Is it also possible to built the Arduino in a product which is mass-produced?Yes - but:

Which possibilities do I have to turn the Arduino in a suitable device?More likely to be cost beneficial to build your own PCB with IO, etc, that meets your requirements.For example, use a wallwart 5V power supply, delete the 7805 regulator, add IO connectors that are more secure than plug-in headers, etc.

As an example of a more robust Arduino derivative, Rugged Circuits offers a version of the UNO with a number of modifications to decrease ESD susceptability, protect the I/O pins from shorts to ground and higher voltages than the stock UNO, and allow it to be powered with upto 24 VDC. I don't think they went so far as to use industrial temperature (?40°C to 85°C) rated components, but since only the ICs and perhaps one or two other parts would need to be changed, it would be possible but somewhat more expensive per board to do so.

For example, use a wallwart 5V power supply, delete the 7805 regulator, add IO connectors that are more secure than plug-in headers, etc.

I would be careful with that advice. Modern switching wall warts should be fine, but if he gets a cheap, old linear based one, it could easily give 8V no load and fry his chips (yes, I made that mistake in my pic days).